Document why ComputedStyle constructor and destructor are protected.

In a previous patch https://codereview.chromium.org/2739553003, the
constructor and destructor of ComputedStyle were moved to be protected,
but it was not clear in the code why this was done. This patch adds a
comment explain it.

BUG=628043

Review-Url: https://codereview.chromium.org/2775153002
Cr-Commit-Position: refs/heads/master@{#459713}
1 file changed